I like to use this analogy when I am talking to students about the value of library resources in the Internet age.
I usually start by asking students what they do for lunch or dinner when they only have five minutes to eat and five dollars. Every time somebody has said, "Go to McDonalds." I then ask, “Is it healthy?” and the students will usually laugh and shake their heads, “No.” Sometimes somebody will say “You can get a salad,” and I will point out that there are some good things on the web, but for the most part, the free web is like McDonald’s. It’s fast, cheap, and easy, but is unhealthy for your academic career. The free web is fine for everyday things, but you should use library sources for things that matter.